1. Field of the Invention
The invention relates in general to a duplex scanner capable of calibrating the two-sided documents, and more particularly to a duplex scanner capable of effectively calibrating the light information of the two-sided documents.
2. Description of the Related Art
With the rapid evolution of computer technology, consumers increasingly demand quality scan images; therefore, producers endeavor to improve the quality of scan image in terms of image resolution and accuracy. Referring to FIG. 1, which depicts the conventional flatbed scanner. The flatbed scanner 100 has a case 102, and the to-be-scanned document (not shown in FIG. 1) is placed on the transparent platen 104. A lower carriage 106 is installed inside the case 102, and a light source 108 is equipped on the lower carriage 106. The flatbed scanner 100 also has the driving device 110 to move the lower carriage 106 forward and backward in a direction parallel to Y-axis. Additionally, a connecting apparatus 112 is equipped for connecting the lower carriage 106 and the driving device 110.
When the scanner starts to scan, the driving device 110 transmits the lower carriage 106 and the light source 108 by the connecting apparatus 112. In the meanwhile, light is first emitted by the light source 108 and then reflected by the to-be-scanned document on the transparent platen 104. Subsequently, light reflected from to-be-scanned document is further reflected by several reflectors 114, and focused on the photoelectric sensing device 118 by the lens 116. Afterward, light received by the photoelectric sensing device 118 is converted into electronic signals. Then, the flatbed scanner 100 further deals with the electric signals to complete the image capture.
The photoelectric sensing device can be any device capable of converting the light signal into the electric signal, such as a Charged Coupled Device (CCD), or a Contact Image Sensor (CIS). The driving device 110 can be any device capable of transmitting the lower carriage 106, such as a leather belt, a steel belt, or the like. The driving device 110 is further connected to a transmitting apparatus (not shown in FIG. 1) such as the assembly of reduction gears and a motor.
FIG. 2 depicts a top view of the conventional flatbed scanner in FIG. 1 during the calibration of the light beam information. Please also refer to FIG. 1. Before scanning the to-be-scanned document, calibration of the light beam information, such as color calibration, brightness calibration, and contrast calibration, have to be performed. In FIG. 1, a lower calibration paper 120 is further installed in the flatbed scanner 100, and fixed on the transparent platen 104. The lower calibration paper 120 extends along the X-direction, like the lower carriage 106. Besides calibration of the light beam information, the lower calibration paper 120 functions in various ways, such as calibration of a start-to-scan point.
Color calibration is taken as an example to illustrate the application of the conventional scanner. The lower calibration paper 120 has either a white region as standard white, or a black region as standard black for the flatbed scanner 100. The standard white is illustrated herein. When the scanner performs color calibration, the driving device 110 drives the lower carriage 106 through the connecting apparatus 112, and the lower carriage 106 is moved under the lower calibration paper 120. Then, the lower carriage 106 starts to scan a scan line 202 on the lower calibration paper 120, wherein the scan line 202 is parallel to the lower carriage 106. Light is first emitted by the light source 108, and then reflected by the lower calibration paper 120. Subsequently, light focused on the photoelectric sensing device 118 is converted into analog electronic signals. The analog electronic signals corresponding to the standard white is further converted to the standard white digital value. Each detecting unit of the photoelectric sensing device 118 is corresponded to a standard white digital value; however, the corresponding standard white digital value of each detecting unit is not exactly the same.
During image scanning, the digital value corresponding to each pixel of the to-be-scanned document is obtained by comparing the analog electronic signal corresponding to the to-be-scanned document with the analog electronic signal corresponding to the standard white. In order to improve the quality of scanning image, the digital value corresponding to the scanning image near the ends of the to-be-scanned document is needed to be compensated by the method of digital signal processing due to the dim lightness of the ends of the light source 108.
However, the adhesive, such as glue, for adhering the lower calibration paper 120 to the transparent platen 104 is likely to stain the lower calibration paper 120. Also, the impurities, such as dust, are likely to contaminate the lower calibration paper 120. The intensity of the light reflected by the lower calibration paper 120 is therefore affected, and the error of color calibration is occurred. To reduce such kind of error, the general solution is making the lower carriage 106 scan numerous scan lines (such as 50) on the lower calibration paper 120 so that more standard white digital values can be obtained, and the average standard white digital value obtained by averaging these standard white digital values.
Referring to FIG. 3, which depicts the conventional duplex scanner. The duplex scanner is the scanner capable of scanning the two-sided document. The automatic document feeder (ADF) 302 is usually installed above the conventional flatbed scanner 100. When the to-be-scanned document (not shown in FIG. 3) on the sheet input plate 304 of the automatic document feeder 302 is send into the scanner, it is transmitted along the paper guide (not shown in FIG. 3) and send to the sheet output plate 306 after scanning. When the to-be-scanned document is transmitted along the paper guide, an upper carriage 308, permanently fixed to the automatic document feeder 302, is scanning one side of the document, and the lower carriage 106 under the document is scanning the other side of the document. In this case, the upper carriage 308 is always installed in the automatic document feeder 302.
Similarly, before the upper carriage 308 starts to scan, a calibration procedure is needed. Color calibration is taken as an example herein. Light is first emitted by the light source 310 at the upper carriage 308 and then reflected by the upper calibration paper 314 fixed on the transparent platen 104. Subsequently, light reflected from the upper calibration paper 314 is further reflected by several reflectors 318, and focused on the photoelectric sensing device 316 of the upper carriage 308 by the lens 320. Afterward, light received by the photoelectric sensing device 316 is converted into analog electronic signals. The analog electronic signals corresponding to the standard white of the upper calibration paper 314 are further converted to the standard white digital value corresponding to the upper carriage 308.
However, in the scanner 100 having the automatic document feeder 302, the upper carriage 308 and the upper calibration paper 314 is permanently fixed to the automatic document feeder 302 and the transparent platen 104, respectively. The poor condition certainly occurs that the upper carriage 308 is not able to scan numerous scan lines on the upper calibration paper 314 due to the lack of relative motion. Therefore, the upper carriage 308 always scans the certain scan line on the upper calibration paper 314 and the average standard white digital values cannot be obtained. If the upper calibration paper 314 is stained or contaminated, the error of the standard white digital values is increased, and the quality of scanned image is seriously affected.